1. Field of the Invention
This invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device for images.
2. Description of the Prior Art
Semiconductor memories have been widely used in recent years for office automation equipments including personal computers. Among such semiconductor memory devices, MOS random access memory (RAM) can be used as the memory for displaying images on a display unit of a personal computer and the like.
A memory device is provided between a CPU and a display unit in a personal computer so that image data may be random accessed by CPU. In a semiconductor memory such as this, addresses in a memory region are provided to correspond to pixels on the display unit. For instance, each of the horizontal scanning lines corresponds to a row address while each bit (pixel) on the horizontal scanning line corresponds to a column address.
In the image processing operation, all of the bits on a horizontal scanning line are sometimes made 0 or 1 by such operation as clearing of the display screen. In this case, the same data should be written in for all the bits of each horizontal scanning line.
However, as the semiconductor memory device is random accessed in the prior art, data writing should be conducted in the unit of bit on a horizontal scanning line. In other words, the same data of 0 or 1 should be written in different cycles respectively for the plural memory cells under the corresponding word in the memory. When a command such as "clear" which should have an access to all the bits on a horizontal scanning line is given, the prior art memory requires a long time to process it, proving to be quite inconvenient.